High-frequency induction heating system



Oct. 5, 1948. H. w. ANDERSON HIGH-FREQUENCY INDUCTION HEATING SYSTEM Filed Oct. 20, 1944 1 IIIIJ... I

Inventor: I Har IdWAnder-son,

His Att orn ey.

Patented Oct. 5, 1948 HIGH-FREQUENCY INDUCTION HEATING SYSTEM Harold W. Anderson, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application October 20, 1944, Serial No. 559,531

3 Claims.

My invention relates to high frequency heating systems, more particularly. to induction heating coil apparatus energized with very high frequency current for heating high resistivity materials, and has for its object a simple, reliable and effi- ,cient induction heating coil means arranged to minimize undesirable Localized heating by the electric field as distinguished from the magnetic field.

In the heating of various materials inductively by a heating coil supplied with very high frequency current, such as a frequency of 80 to 100 megacy-cles, high voltages are applied to the terminals of the heating coil, even though the coil has only a few turns. Ihese high voltages between the end turns of the coil and between portions of the coil produce an electric field in addition to the magnetic field produced by the current in the coil with localized heating caused by the electric field. A sandwich consisting of a frankfurter in a roll heated inside a conventional 'helical heating coil supplied with very high frequency current showed localized heating and burning of the bread at the ends of the roll adjacent the two end turns of the coil.

An explanation of this localized heating at the ends of the frankfurter is that the frankfurter, while a so-called high resistivity body, is a good electric conductor as compared with the dielectric material constituted by the bread. As a result,

the frankfurter assumes an intermediate voltage throughout its length, and consequently one half of the coil terminal voltage appears between each end of the coil and the adjacent end of the frankfurter. An electric field is therefore produced between each end turn and the adjacent end of the frankiurter, and this field converges on any projection on the ends of the frankfurter with an accompanying high voltage gradient in the field and arcing between the projection and the bread. Likewise, an air pocket between the bread and the franltfurter offers a path of relatively low dielectric value as compared with the bread for the electric field with concentration of the field in the air pocket and arcing. Very substantial burning of the bread is caused by the arcing.

In carrying out my invention, I provide induction coil means having the high potential end turns of the coil in the middle of the heated article so that the ends of the article being heated, where projections and air pockets are most likely to be found, are not exposed to these high voltage parts. Hhus, in the heating of a frankf urter sandwich, the central portion of the sandwich is exposed to the high potential parts of the coil 2 whereby arcing and burning of the bread is pre-- vented because of the uniform and smooth configuration of the f-rankfurter in the middle and the consequent elimination of projections and air pockets.

In one form of my invention, I provide two in duction heating coils arranged in end-to-end juxtaposition and wound in the same direction, which coils in effectform a conventional helix broken at its center to form two middle terminals. I energize the two coils by supplying high frequency currents of opposite phase to the two inner terminals, a common connection of the supply source being connected to the two outside terminals of the two coils so that the two coils produce cumulative high frequency fields. Preferably I use an electronic oscillation generator of the push-pull type having a grounded central tap or terminal, the two outside ends of the coils being connected together through the direct current power supply to ground.

For a more complete understanding of my invention, reference should be had to the accompanying drawing, Figure 1 of which is a diagram,- matic representation of electric high frequency heating apparatus embodying my invention and Figure 2 of which is a section along line 22 of Figure 1.

Referring to the drawing, I have shown my invention in one form as applied to the heating of a sandwich consisting of a bread roll i and a frankiur-ter 2. As shown in the drawing, the roll is split by a lengthwise out along one side nearly separating the two halves of the roll which are compressed around the frankfurter placed between them. Also, as shown, the roll is substantially longer than the frankfurter so that each end of the roll projects beyond each end of the frankfurter. The roll and irankfurter preferably are provided with a wrapper of Cellophane or waxed paper (not shown). For heating, the sandwich is inserted in a heat refractory electrically insulating tubular member 3, preferably made of Pyrex glass, on whichis wound two coaxial helical coils 4 and 5. Each coil consists, as shown, of substantially one and one-half turns, although it will be understood that any suitable number of turns may be used; The two coils are wound in the same direction around the tube and they are arranged in end-to-end juxtaposition so as to produce magnetic fields which are coextensive in the region between their two adjacent or inner ends.

High frequency electric current is supplied to the coils by means of a push-pull electric discharge device oscillator generator, producing equal voltages opposite in phase, connected to the two adjacent inner coil terminals 6 and 1. This high frequency generator, as shown, comprises two electric discharge devices 8 and 9 having their anodes or plates connected respectively to the coil terminals Band I. The two cathodes l and I l are electrically connected together and to a ground connection [2. The discharge devices are provided with grid excitation by means of suitable inductances l3 and I4 connected to the grids of the discharge devices and having their opposite ends connected through a grid leak resistor I6 in parallel with a capacitor l to the ground l2. Direct current at a suitable high voltage is supplied from the positive supply main 20 through a high frequency choke coil I1 to the two outside ends of the two coils forming the heating inductor. The negative direct current supply main [8 is connected to a ground connection l9.

Thus it will be observed that the electric discharge devices are connected to the two inner terminals 6 and 1, whereas the two outside terminals of the coils are connected through the direct current supply source to the ground I9. I have found that this heating coil apparatus gives effective heating of the frankfurter with appreciable heating of-thebread but without burning of the bread. The frankfurter is heated by currents induced init'by the magnetic field produced by the electric currents in the coils, while the bread is heated by the electric field. To avoid burning of the bread, only frankfurters of smooth configuration between their ends should be used and the two halves of the split roll should be compressed firmly around the frankfurtcr so as to avoid air pockets between the bread and the frankfurter.

In a typical apparatus, the tube 3 had a diameter of approximately 2% inches, while the overall length of the two coils, i. e., between the upper and lower outside terminals, was approximately 4 inches. The turnsof each coil were approximately inch apart. A direct current voltage was supplied to the generator of approximately 2800 volts "and high frequency current was generated having a frequency of approximately 80 megacycles at 5500 volts. With 850 watts .power supplied by the generator to the heating coils, the frankfurter was heated to a boiling temperature in'about 20 seconds.

It will be understood'that'the frankfurter, because of its low dielectric value as compared with the bread and air, assumes a uniform potential i throughout .its length with respect to the electric field. Thus,'in the example described, with an instantaneous peak voltage of 5500 volts between the terminals '6 and I, substantially one half of this voltage, or 2750 volts, appears between each terminal 6 and I and the frankfurter and produces electric fields of greatest density in the shortest path-between each terminal and the frankfurter. But since in these intermediate regions no projections on the frankfurter or air pockets are present to cause a concentration of the electric field, arcing and burning of the bread is less likely to occur.

The currents in the two coils, while displaced 180 degrees in phase with respect to each other, produce magnetic fluxes which are in phase and cumulative. This is because the coils although wound in the same direction on the tube 3, are oppositely wound with respect to the two terminals 6 and 1. mother words, if the two'terminals 6 and 1 were electrically connected together to form a single coil and a supply source connected to the two terminals of this coil, then it will be understood that the currents in the terminals 6 and 1 would be oppositely directed, i. e., when the current is flowing toward one coil, it will be flowing away from the other coil. Thus, the two coils 6 and 5 form in effect a single coil or heating inductor which is broken at its center to form two terminals to which a push-pull source of high frequency is connected.

It will be understood that the two coils 4 and 5, connected in series with each other for the flow of high frequency oscillatory current, are reson 'ated with the combination of the distributed capacitance of the conductors and the interelectrode capacitance of the electric discharge devices to form a high frequency oscillation circuit.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto since many modifications may be made, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. High frequency heating apparatus comprising two helical induction heating coils wound in the same direction, means supporting said coils in end-to-end adjacent relation, electric discharge device oscillation generator means connected to the inner adjacent ends of said coils, a connectionelectrically connecting together theouter ends of said coils thereby to form a high frequency oscillation circuit including said generator means and said coils in series with one another for the fiow of high frequency current therethrough whereby said coils produce cumulative magnetic fields and an electric field having its greatest density adjacent the inner ends of said coils for the heating of an article inserted into said coils, a chokecoil, and connections for supplying direct current to saidgenerator means through said choke coil.

2. High frequency heatingapparatus comprising two helicalinduction heating coils wound in the same direction, means supporting said coils in end-to-end adjacent relation, two grid controlled electric discharge devices having their plates connected respectively to the inner adjacent ends ofsaid coils and their cathodes electrically connected together to form a push-pull high frequency oscillation-generator, a connection electrically connecting together the outer ends of said coils thereby to form a high frequency oscillation circuit including said discharge devices and said coils in series with one another for the flow of high frequency current therethrough whereby said coils produce cumulative magnetic fields and an electric field having its greatest density adjacent the inner endsof said coils for the heating of an article inserted into said coils,

a choke coil, connections including said choke coil for supplying direct current to said discharge devices,,said supply connections being connected to saidconnection between the outer ends of said coils and to said connection between the oathodes of said discharge devices, and a ground connection for one of said supply connections.

3. High frequency heating apparatus for heating an elongated article comprising two helical induction heating coils wound in the same direction, means supporting .saidcoils in end-.to-end trically connecting together the outer ends of said coils thereby to form a resonant high frequency oscillation circuit including said discharge devices and said coils in series with one another for the fiow of high frequency current therethrough whereby said coils produce cumulative magnetic fields and an electric field having its greatest density adjacent the inner ends of said coils for the heating of an article inserted into said coils, the juxtaposition of the inner ends of said coils being such that the fields between said coils are coextensive so that in effect said coils constitute a single helical heating coil broken substantially at its center for the connection of said oscilla tion generator, a choke coil, a source of direct current supply for said discharge devices having its positive terminal connected through said choke coil to said connection between the outer ends of said coils and its negative terminal grounded, and a ground connection for said connection between the cathodes of said discharge devices.

HAROLD W. ANDERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

